1. Field of the Invention
The present invention relates to an improved catalytic distillation process for the production of alkylated aromatics from the alkylation reaction of olefin and aromatic compounds.
2. Description of the Related Art
The advantages of the catalytic distillation process over conventional liquid phase alkylation processes are well recognized. See, for example, U.S. Pat. Nos. 4,307,254, 4,443,559, 4,849,569, to Smith, Jr., U.S. Pat. No. 5,243,115 to Smith, Jr. et al., and U.S. Pat. No. 4,439,350 to Jones, Jr. et al.
Some of the most common alkylation reactions include the alkylation of benzene with either ethylene or propylene to produce ethylbenzene or cumene, respectively. Ethylbenzene is particularly important for its use in the production of styrene, a precursor to polystyrene, while cumene is particularly important for its use in the production of phenol and acetone.
The catalytic distillation units thus far used are not, by themselves, capable of the complete conversion of the olefin and aromatic reactants to alkylated aromatic products. Accordingly, the catalytic distillation process typically includes an alkylation finishing reactor, operated in the liquid phase, for converting any remaining unreacted olefin and aromatic compounds to alkylated aromatic products with nearly complete conversion of the olefin. See published U.S. application Ser. No. 2004/0254412 to Pohl.
While the catalytic distillation method for alkylation reactions has provided many benefits, there remain several areas in need of improvement. For example, the process is known to suffer from a lack of reaction efficiency (i.e., impeded olefin conversion). This impedance of olefin conversion is primarily caused by an inability to control the required olefin partial pressure in the alkylation unit.
In particular, the heat of reaction (the reaction occurring in the liquid phase over the catalyst) causes partial vaporization of aromatic compounds, and a significant increase in the vapor rate from the lowermost portion of the catalyst to the uppermost portion. Gaseous olefin, flowing counter-currently upwards through the reactor, is absorbed into the liquid phase and is consumed by reaction over the catalyst. Since vapor-liquid equilibrium is approximately maintained, the result is a continuous reduction in olefin partial pressure and a corresponding decrease in liquid phase olefin concentration from the lowermost portion of the catalyst to the uppermost portion. This reduction in liquid phase olefin concentration causes correspondingly lower reaction rates and requires an ever-greater amount of catalyst to maintain the same incremental olefin conversion as it proceeds up the reactor.
Since the cost of the catalyst is typically significant, larger amounts of catalyst can result in a significant capital investment for the process. In addition, a larger amount of catalyst further exacerbates the capital and operational costs of the process by requiring a larger alkylation reactor for housing the catalyst.
Some degree of control of olefin partial pressures have been achieved in the art, but these have not resulted in maintaining satisfactory olefin conversion rates in all catalyst beds. For example, improved olefin conversion rates have been achieved throughout the catalytic distillation unit by employing an optimal benzene vapor feed rate to the bottom of the catalytic distillation unit in combination with controlling the number of olefin injection points and the flow rate to each injection point. However, even with this improvement, olefin conversion rates still drop off sharply in the middle to upper catalyst beds.
There is a need, therefore, for an improved catalytic distillation process for alkylation reactions with an improved conversion rate of the olefin. There is a particular need for improving the conversion rate of the olefin by better maintaining a desired olefin partial pressure throughout the catalyst of the catalytic distillation unit. Such an improvement would allow for the use of lesser amounts of catalyst, and consequently, a reduction in size of the alkylation reactor, and/or greater overall olefin conversion across the alkylation reactor.